The links below are more from the TAC perspecitive but can be a place to start.
http://www.cisco.com/warp/public/788/voip/voip.shtml
Look for the section on gatekeepers.
I will try and give you a brief explanation from the 10,000ft view. Gatekeepers are like a match maker. Each gateway registers with him and he knows what numbers they can provide access to. So when a gateway send a call to the gatekeeper he knows which other gateway can handle that call. He sends that info back to the orginating gateway and they talk to each and make the call happen. Not a very good explanation but hopefully it made some sense.
Thank you,
-Mckee
